The most common and most likely answer to this crossword clue is the 6 plus 8 letter word SPRING CLEANING
I've seen this crossword clue in The Daily Telegraph
S
P
R
I
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G
C
L
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A
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Buddy explains!
The straight part
We know that "seasonal brush-up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case seasonal brush-up = spring cleaning.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SPRING CLEANING mean?
SPRING CLEANING noun- A systematic cleaning of a residence at the end of winter
- (by extension) A systematic cleaning, reorganization, or weeding out of a system